LibJS: Remove the JS_TRACK_ZOMBIE_CELLS option

This feature had bitrotted somewhat and would trigger errors because
PrimitiveStrings were "destroyed" but because of this mode they were not
removed from the string cache. Even fixing that case running test-js
with the options still failed in more places.

Userland/Libraries/LibJS/Heap/Cell.h

@@ -24,18 +24,9 @@ public:
     bool is_marked() const { return m_mark; }
     void set_marked(bool b) { m_mark = b; }
 
-#ifdef JS_TRACK_ZOMBIE_CELLS
-    virtual void did_become_zombie()
-    {
-    }
-#endif
-
     enum class State {
         Live,
         Dead,
-#ifdef JS_TRACK_ZOMBIE_CELLS
-        Zombie,
-#endif
     };
 
     State state() const { return m_state; }